The present invention relates to a fluid-rotating apparatus and more particularly to a fluid-rotating apparatus, such as a positive displacement vacuum pump used to discharge gas from a vacuum chamber of a semiconductor-manufacturing apparatus, having a construction for synchronously rotating a plurality of rotary shafts at a high speed.
A vacuum pump for generating a vacuum environment is required by a CVD apparatus, a dry etching apparatus, a sputtering apparatus, an evaporating apparatus and the like to be used in the process for manufacturing semi-conductors. The vacuum pump is also used in the process of manufacturing a magnetic disk or a liquid crystal display.
In a positive displacement vacuum pump, two rotors having screws formed on the peripheral surface thereof are synchronously rotated with the screws engaging each other. The suction and compression of gas are repeated by changing the volume of a space formed between the screws so as to discharge gas. Each rotor is provided with a driving motor which is electrically controlled to rotate the rotors synchronously. A servo motor, the rotational speed of which can be freely controlled is used as the driving motor.
In the positive displacement vacuum pump, unless a plurality of motors, namely, a plurality of rotary shafts is synchronously rotated, the rotors collide with each other. As a result, a desired pumping operation cannot be performed and power is wastefully consumed and constituent components such as the rotors are damaged.
According to the conventional positive displacement vacuum pump, reference pulses are supplied from the same pulse generator to the control circuit of each motor for driving each of the rotary shafts. Each control circuit controls the rotation of each motor, namely, the rotation of each rotary shaft in conformity with the reference pulses. In this manner, the rotations of a plurality of rotary shafts are synchronized.
In recent years, there has been a strong demand for the development of a vacuum pump having a high operational performance due to the rapid processes of the process for manufacturing semiconductors. It is desirable for the processes for manufacturing semiconductor to become more and more highly integrated, to be able to increase the diameter of a wafer and vertically enlarge the wafer, and to be able to manufacture many kinds of semiconductors in small quantities. For high integration, it is necessary for the equipment to be clean. For the development of a wafer with a large diameter, the area occupied by the equipment must be small. For vertical enlargement of the wafer, complex processing (multi-chamber) is required. In order to manufacture many kinds of semiconductors in small quantities, the equipment must have a kind of network.
In order to comply with the above demands, it is necessary for the vacuum pump to be prevented from being polluted by oil or the like, to generate a high vacuum, to be corrosion-resistant, and to have a high efficiency per space.
Above all, the vacuum pump is required to generate vacuum over a wide range. Although a degree of vacuum as high as 10.sup.-8 to 10.sup.-10 torr is required in recent years, one vacuum pump is incapable of generating such a high vacuum. That is, a positive displacement vacuum pump called a roughing rotary pump is suitable for discharging gas in a viscous flow region, the pressure of which is almost equal to atmospheric pressure, but the degree of vacuum obtained by the vacuum pump is in the range from atmospheric pressure to a low vacuum degree of 10.sup.-3 torr. According to a kinetic vacuum pump called a turbo pump, one rotor imparts momentum to gas molecules by its rotation so that they are transported by the momentum. As a result, the gas is discharged from the vacuum chamber. The turbo pump generates a vacuum degree as high as 10.sup.-2 to 10.sup.-10 torr, but in principle, the turbo pump is capable of discharging gas from the vacuum chamber only in a molecular flow region, the vacuum degree of which is greater than 10.sup.-1 torr and smaller than 10.sup.-3. In order to obtain a high vacuum of 10.sup.-8 to 10.sup.-10 torr, it is necessary for a degree of vacuum of 10.sup.-2 to 10.sup.-3 torr to be generated by the rotary pump and then, for a predetermined high vacuum to be obtained by the turbo pump.
The use of two types of vacuum pumpleads to the installation of large equipment. That is, in order for equipment to accomplish composite processing (multichamber), each chamber is required to be equipped with an evacuating device. The use of two types of vacuum pumps for each chamber does not allow the entire evacuating apparatus to be compact. Consequently, space cannot be efficiently used and in addition, the equipment costs are high.
Accordingly, the conventional method is capable of rotating a plurality of rotary shafts at approximately the same number of rotations or at the same rotational speed, but is incapable of accurately synchronizing the rotational angles of a plurality of rotary shafts or phases thereof with each other.
In the positive displacement vacuum pump, a pair of rotors is required to rotate at a constant speed with a backlash maintained between screws of the rotors so that the rotors do not contact each other. Even though the position of each rotor is accurately set so that they do not contact each other in assembling the fluid-rotating apparatus, there is a possibility that the rotational positions of the rotors will become dislocated from a set rotational position during operation due to the difference between the inertial masses of the rotors, loads applied thereto or the performance of the motors. There is a great possibility that the rotational positions of the rotors will be dislocated from the set rotational position during acceleration of the rotors, during steady rotation thereof, and during the deceleration thereof.
According to the conventional method, the rotation characteristics of the rotary shafts are differentiated from each other because of the difference between loads applied thereto and inertial masses thereof even though the same reference pulse is supplied to the control circuit of each motor. Thus, the rotors cannot be prevented from becoming dislocated slightly from the set rotational position.
FIG. 7 shows an example of another conventional positive displacement vacuum pump in which screws are formed on rotors. Two rotors 412 are provided in a housing 411 with the rotary shafts thereof parallel with each other. Screws are formed on the peripheral surface of each rotor 412. The recesses (grooves) 413a of one rotor 412 engage the projections 413b of the other rotor 412, thus forming a space therebetween. With the rotation of the rotors 412, the volume of the space is changed to perform operations for suction and discharge of gas.
In the above conventional positive displacement vacuum pump, timing gears 416 are provided as a means for rotating the two rotors 412 synchronously. That is, the rotation of a motor 415 is transmitted to one of the timing gears 416c engaging each other and disposed on the shaft of each rotor 412 via a driving gear 416a and an intermediate gear 416b. The phase of the rotary shaft of each of the rotors 412 is adjusted by the engagement between the timing gears 416c. In this type of vacuum pump, the gears are used to transmit the power of the motor 415 and rotate the rotors 412 synchronously. Consequently, lubricating oil filled in a mechanical-operating chamber 417 accommodating the gears is supplied to the gears. In addition, a mechanical seal 419 is provided between the mechanical-operating chamber 417 and a fluid-operating chamber 418 accommodating the rotors 412 so as to prevent the lubricating oil from penetrating into the fluid-operating chamber 418.
This type of vacuum pump, with the above-described construction, having two rotors has the following drawbacks:
1). Many gears are required to transmit the power of the motor and rotate the rotors synchronously. Therefore, the vacuum pump is composed of many parts and thus its construction is complicated. PA1 2). Gears which rotate in mesh are required to rotate the rotors synchronously. Accordingly, the rotors cannot be rotated at a high speed and the vacuum pump is large. PA1 3). It is necessary to replace the mechanical seal periodically when it becomes worn, namely, it is not maintenance-free. PA1 4). Since the sliding torque of the mechanical seal is great, mechanical loss is large. PA1 characterized by further comprising: PA1 a PLL control circuit for rotating the driven side rotary shaft synchronously with the driving side rotary shaft under PLL control operation; and PA1 a gain-switching means for setting a great gain in the PLL control circuit when the driving side rotary shaft is accelerating or decelerating, and setting a small gain therein when the driving side rotary shaft is rotating in a steady state.
In order to overcome these drawbacks, there has been proposed a positive displacement vacuum pump comprising a plurality of rotors, each driven by an independent motor and rotated synchronously by a method using a detecting means such as a rotary encoder for detecting the rotational angle of each rotor and/or the number of rotations thereof. This vacuum pump does not include components such as gears which are operated in sliding contact with each other. The vacuum pump allows the rotors to be rotated at a high speed synchronously, eliminates maintenance, is clean, and is compact.
In the proposed vacuum pump, each rotor is provided with an independent motor which is electrically controlled to accomplish the synchronous rotation of the motors. A servo motor, the rotational speed of which can be freely controlled, is used as the motor.
As described previously, in the positive displacement vacuum pump, unless a plurality of motors, namely, a plurality of rotary shafts is synchronously rotated with a high accuracy, the rotors collide with each other. As a result, a desired pumping operation cannot be performed and power is wastefully consumed and constituent components such as the rotor are damaged. The synchronous rotation means that the rotational speeds of a plurality of rotary shafts coincide with each other and the rotational positions thereof, namely, the phases thereof coincide with each other.
In order to control the operation of a plurality of rotary shafts, phase locked loop (PLL) control is used to control the operation of driving motors so that the phase of a reference frequency used as the reference of the rotation of the rotary shaft coincides with the phase information of the rotary shaft detected by an encoder mounted on each rotary shaft.
In the PLL control, the oscillation frequency of a crystal oscillator is used as a reference frequency. A frequency to be compared with the oscillation frequency is detected by a frequency generator coaxial with the motor so as to control (compare) the phases of both frequencies. In this manner, the stabilized rotational speed of the motor can be obtained. The rotations of a plurality of rotary shafts are synchronized by supplying the same reference frequency to a PLL control circuit which drives the motor for rotating each rotary shaft.
According to the above-described method for controlling the drive of the rotary shafts of the positive displacement vacuum pump, the gain of the control circuit is set equally at the operation start point of the rotary shaft, during deceleration, at its stop point, and its during steady operation. Therefore, electric power is wastefully consumed. The reason is as follows:
It is necessary to make the power of the motor large in order to accelerate and decelerate the rotation speed of the rotary, shaft greatly. Even a slight degree of disturbance influences the rotation the rotary shaft. For example, if a load fluctuates, the rotational speed of the rotary shaft may be changed. In the positive displacement vacuum pump, it is necessary to synchronize the rotations of the rotary shafts with each other during acceleration deceleration. Therefore, the level of speed instruction signals supplied to the control circuit of each rotary shaft are changed equally. But the rotations of the rotary shafts become asynchronous owing to the fluctuation in the rotational speed and rotational position thereof caused by an external factor. It is necessary for the control circuit of each rotary shaft to reliably control the rotation of the rotary shaft so that the rotations of all the rotary shafts are synchronized. To this end, the gain (amplification factor) of the control circuit should be great. With a great gain, the rotational speed and position of the rotary shaft fluctuated by a disturbance can be restored to the original correct state by a great power.
However, electrical energy is increasingly consumed with the increase of the gain and thus a large amount of electric power is required.
Since the inertia effect works during steady operation of the rotary shaft, the synchronous rotation of a plurality of rotary shafts can be maintained even though a disturbance is applied thereto. Accordingly, during steady rotation of the rotary shaft, it is unnecessary to use a control circuit in which a large gain has been set. The use of the control circuit in which a great gain has been set is a wasteful consumption of electrical energy.
Unlike with a fluid-rotating apparatus in which the rotary shaft is started and stopped frequently and is accelerated and decelerated greatly, electrical energy is wastefully consumed and running costs are high in fluid-rotating apparatus which are operated mainly in a steady operation according to the conventional control method. The positive displacement vacuum pump described previously is operated mostly in the steady operation.